Chrome

1. Unzip the downloaded file.
2. Activate the developer mode under your extensions settings to be able to load the extension.
3. Load the extracted extension folder or the manifest.json file.

Edge

1. Unzip the downloaded file.
2. Activate the developer mode under your extensions settings to be able to load the extension.
3. Load the extracted extension folder or the manifest.json file.

https://docs.microsoft.com/en-us/microsoft-edge/extensions-chromium/getting-started/extension-sideloading

Firefox

1. Go to about:config (enter it into address bar).
2. Set xpinstall.signatures.required to false.
3. Go to about:addons.
4. Drag & drop the extension zip file or click on the & choose install add-on from file.